Etching tungsten coils



concentration for Patented Sept. 4, i951 2,566,615 ETCHING TUNGSTENCOILS Gerald W.

Boston, .Mass.,

Keilholtz, Lynn, and Martha J Bergin,"

assignors to Sylvania Electric Products Inc., Salem, Mass., acorporation of Massachusetts No Drawing. Application March 21, 1947, ISerial No. 736,414

3 Claims.

This invention relates to the reduction of the diameter of tungstenelectric lamp coils and more particularly to a chemical etching processto effect said reduction.

An obj ect of this invention is to provide a process for producingtungsten wire which is finer than that which can be drawn commerciallyby mechanical means.

Another object of this invention is to provide a process for treatingelectric lamp tungsten coils which will make it possible to decrease thewattage thereof.

A further object of this invention is to provide a process for reducinthe diameter of tungsten wire without mbrittling it.

In. attempting to provide tungsten wire of smaller diameter than thatwhich can be drawn commercially, it has been the practice to etch thewire to reduce the diameter thereof to the desired size. However, wehave found that the etching process weakens the wire considerably. Theembrittled wire is very difficult to handle in the formation of electriclamp coils therefrom and, as a result, excessively high losses aresuffered. To overcome this, we have developed a process for reducing thediameter of drawn tungsten wire after it has been formed into coils forelectric lamps.

In the process of our invention tungsten coils are treated in analkaline oxidizing solution under controlled conditions of temperatureand a definite time interval. Initially, the coils are wound with moreturns per inch than is normally done and the etching process producesthe desired final spacing as the diameter of the wire decreases.

In carrying out the process of our invention, a plurality of electriclamp coils of tungsten wire are disposed in a bath of an alkalineoxidizing solution. For our purposes we have found that alkalinepotassium ferricyanide gives very desirable results. However, otheralkaline oxidizing solutions may be used without departing from thespirit of our invention. For example, satisfactory results may beobtained with potassium permanganate and ceric salts.

The concentration of the bath may be varied depending on the speed withwhich it is desired to complete the etching process and the wirediameter reduction desired. For example, in reducing coils formed fromtungsten wire of the order of .3 mg. per 200 m. m. 'to about .18 mg. wehave used a bath comprising about grams of KsFe(CN) 6, about 40 grams ofNaOH and about 350 ml. of H20.

'care should be taken We have found it desirable to maintain thetemperature of the bath at about 20 C. The temperature of the bathshould be carefully controlled because an increase in the temperature ofthe bath speeds the etchin process. If one permits the process todevelop too rapidly, it is quite dirllcult to accurately control thereduction of the wire diameter. However, we have found that it isdesirable to etch the coils faster than tungsten wire is normally etchedbecause it reduces the formation of fissures which tend to form in slowetching along crystal boundaries.

For example, We have found that about a 50% reduction may be effected inabout six minutes when coils of the size mentioned above are treated ina bath of the above-mentioned concentration.

After the coils have been disposed in the solution, the bath is agitateduntil all the coils are off the bottom of the container and swirlingwith the etching solution. This condition is maintained for the desiredtime, i. e., until the coils have been reduced in diameter to the extentdesired.

After the etching has been completed, the bath agitating means isdisconnected, the coils are permitted to settle in the bottom of thecontainer and the etching solution is decanted. Distilled water is thenpoured over the coils and they are washed thoroughly, preferably twice.After the second washing, the coils are poured, with the aid 01' morewater, into a Gooch crucible. The crucible is then placed in a bath ofNHrOI-I for about five minutes to remove oxide from the coils. Thecrucible is then dipped twice into baths of hot distilled water andfinally rinsed thoroughly in acetone. "lhe coils are then dried in anoven at about C. for about 15 minutes.

We have found that the process of our invention enables us to wind coilsto more turns per inch than normal coils. By winding about 2000 to about2200 'l. P. I. it is possible to etch coils without tangling orintermeshing. The etching process reduces the diameter of the wire sothat the finished product has the proper spacing between coil turns.

In order to insure uniform etching of the coils, to adjust the speed ofthe bath agitating means so that the coils do not touch each other orthe sides of the container. We have found that. due to the speed withwhich the process of our invention is accomplished, this point is quiteimportant. The process of our invention can be satisfactorily employedonly when 3 the coils are uniformly in contact with the solution.

Precise control of the etching process is desirable because variation ofthe component concentrations of sodium hydroxide and potassiumferricyanide causes considerable variation in the etching power 01' thesolutions. For example, an increase in the concentration of potassiumferric;y anide increases the etching Dower.

What we claim is: a

l. The process of reducing the diameter oii coiled tungsten wire whichcomprises bathing the coiled tungsten wire in an agitated solution ofalkaline potassium ferricyanide.

2. The process of reducing the diameter oi coiled tungsten wire whichcomprises bathin the coiled tungsten wire in an agitated solution ofalkaline potassium ferricyanide until the diameter of the wire has beenreduced to the desired size, decanting said solution, washing the wirein distilled water, bathing the wire in a solution of NI'IQOH, bathingthe wire in hot distilled water,

. 4 rinsing the wire in acetone. and drying the coils. 3. The process ofmanufacturing filaments for electric lamps from tungsten wire whichincludes the steps of coiling the tungsten wire, bathing the coiled wirein an agitated solution of alkaline potassium ferricyanide, andwashingthe coiled wire to free it from substantially all contaminants GERALD W.KEILHOLTZ.

MARTHA J. BERGIN.

REFERENCES CITED UNITED STATES PATENTS

1. THE PROCESS OF REDUCING THE DIAMETER OF COILED TUNGSTEN WIRE WHICHCOMPRISES BATHING THE COILED TUNGSTEN WIRE IN AN AGITATED SOLUTION OFALKALINE POTASSIUM FERRICYANIDE.